When designing a power supply system, in addition to the performance of the power network, operational and personal safety must be ensured. A protective concept for a faultless operation therefore comprises protective measures which ensure continuous power supply with simultaneous personal and plant protection.
Unearthed mains have proven themselves with regards to operational safety and the minimisation of the risk of unavailability of the power supply. In unearthed mains, there are no active conductors directly connected to earth potential, so that in the event of a (unipolar) earth fault, in the absence of a return wire, a short circuit current, which triggers a fuse and leads to an interruption of operation, cannot flow as it can in earthed systems. An unearthed network can be operated further safely in spite of this earth fault.
Completely insulated, unearthed mains furthermore offer the best possible protection for people, in case they should touch a live conductor, as due to the ideally infinitely large impedance value between conductor and earth, no closed circuit with a current flowing via the (earthed) person can result.
In practice, however, the mains have finite ohmic and capacitive resistances—a complex-valued network leakage impedance—with respect to earth, so that in the event that a conductor becomes connected to earth potential by means of touching an (earthed) person, a closed circuit results via this network leakage impedance and the human body, in which closed circuit potentially lethal leakage currents can flow. The leakage current is principally defined by the capacitance formed by the conductor arrangement and earth and thus has a predominantly capacitive component.
Particularly in hospitals with their multiplicity of medical electrical devices, people come into direct contact with live parts, in that for example the patient is connected via electrodes or probes to a device of this type during an examination or during an operative intervention. An additional current flow, which is brought about unintentionally, could impair the action of the medical measure and, in the worst case scenario, have fatal consequences for the patient. It is for this reason that unearthed mains in hospitals are subject to particular regulations which require a constant checking of the maximum leakage current THC (total hazard current). A monitoring device of this type is also designated as a line isolation monitor (LIM).
In this case, the current which would flow in the event that a person touches a live conductor is designated as the maximum leakage current. In accordance with the definition, this current arises from the division of the maximum occurring voltage between conductor and earth (network voltage ULE,max) and network leakage impedance (ZE): THC=ULE,max/ZE.
The two values ULE,max and ZE required for determining the maximum leakage current can be determined independently of one another. Whilst the determination of the network voltage for the most part results from a conventional voltage measurement by means of a resistor network and does not place any great demands on measurement technology, the determination of the complex-valued network impedance ZE shapes up to be complex, as a suitable test or measurement signal for impedance measurement has a small amplitude and is overlaid by the network voltage and further interference pulses during the continuous monitoring of a conductor system. A fundamental problem therefore consists in the separation of the measurement signal from the line-to-line voltage, whereby the selection of a suitable frequency of the measurement signal can make a decisive contribution to a reliable determination of the network leakage impedance and therefore of the leakage current.
A method is known from the published document U.S. Pat. No. 4,206,398, in which, for determining the leakage current, a measurement signal is impressed into the mains, which, although it has the network frequency, is additionally phase-modulated. Due to the continuous phase modulation of the measurement signal, this can be separated as a non-stationary signal portion from the total voltage by means of the filtering of the useful portion (network voltage). In a closed-loop control circuit, this test voltage is used as a control variable, it being possible to set the impressed current in such a manner that the test voltage corresponds to the network voltage. The stationary end value of the current then corresponds to the leakage current to be determined.
In the digital line isolation monitor (LIM) described in the published document U.S. Pat. No. 5,450,328, a sinusoidal measurement current is impressed into the mains for determining the leakage impedance, its value of the network frequency can be calculated with known modelling of the leakage impedance. After filtering out and measuring the test voltage, which falls as a consequence of the measurement current across the leakage impedance, the value of the leakage impedance can be calculated for the network voltage with known modelling of the leakage impedance.
In the published document U.S. Pat. No. 4,472,676, a system for measuring the leakage impedance is disclosed, in which the test signal fed into the conductor network only has a small frequency deviation from the network frequency. By evaluating the interference pattern of the total voltage resulting by means of the overlaying of test voltage and network voltage, it is possible in the case of known impressed measurement current to measure a voltage drop proportional to the leakage impedance.
The previously mentioned solution approaches show various ways in which the measurement signal in the frequency spectrum can be extracted from the aggregate signal. By means of a phase modulation of a test signal with the same frequency as the network, by means of a test signal of clearly deviating frequency with the option of setting the frequency manually or by means of a test signal with only small frequency deviation and subsequent evaluation of the interference signal.
Common to all three solution approaches is the fact that the frequency of the measurement signal is always constant. As, however, in the practical operation of a conductor network, interference signals, which lie in the frequency range of these fixed measurement frequencies, can also arise to a greater extent during the measurement, inaccurate determinations of the maximum leakage current and thus, in the case of the monitoring of a conductor network, the triggering of false alarms can occur. By way of example for this, mention may be made of the generation of interference frequencies remote from the network frequency by means of the increasing use of frequency converters in the hospital sector or the mutual influence on the network frequency by means of two unearthed mains installed in parallel.